Mobile cellular operators are placing increasing demands on capacity in order to support greater numbers of subscribers and higher bit-rate services. This in turn is placing increasing pressure on the restricted amount of available radio spectrum. In attempts to provide more efficient use of the available spectrum, workers in this field have considered the use of spatial processing and of the provision of micro-cells.
Spatial processing exploits the multi-path characteristics of the mobile radio channel by means of multiple antennas at the transmitter and at the receiver. The benefit arises in two ways. Firstly, there is a diversity gain, which arises provided the antennas at the receiver are sufficiently far apart for the signals they receive to be uncorrelated. Then, the signal received at any one antenna varies independently of the signals received by the other antennas and the signal-to-noise ratio of the combined signal is improved as a consequence. Secondly, the multi-path channel can be separated into independent spatial modes, each of which is capable of supporting traffic in its own right. To exploit this effect requires coding of the signal at the transmitter and multiple transmit antennas. The combination of these two effects, diversity gain and independent spatial modes, leads to an improvement in spectral efficiency that is proportional to the product of the number of transmit antennas and the number of receive antennas. Use of either of these effects alone may still provide a useful improvement, albeit smaller than that realised in combination. The improvement in spectral efficiency is only realised if the antennas are sufficiently far apart for the received signals to be uncorrelated. In practice this means that the antennas must be separated by at least one half wavelength at the frequency of the radio signal, thus restricting the application of spatial processing to higher frequencies and or larger antenna installations. Recent work has sought to overcome this restriction by suggesting that the antennas need not be placed on the same terminal, either at the transmitter or at the receiver, and that groups of terminals can cooperate to form virtual transmit and receive antenna arrays.
Micro-cells are an extension of the cellular concept to smaller cells in an attempt to accommodate larger numbers of users. In cellular radio, the radio spectrum that is used in one geographical area, or cell, may be re-used in other cells, provided that the cells are sufficiently far apart for mutual interference to be below a pre-determined level. The level of interference is dependent on the ratio of the cell diameter and the distance between cells in such a way that it remains constant if both are changed in proportion, for example if both are halved. Hence higher capacity density can be achieved by means of smaller cells. The small cells typical of micro-cellular architectures therefore offer the potential for very high capacity density and hence high overall spectral efficiency. However, such architectures, while offering high capacity density within the cells, pose the difficult backhaul problem of linking the myriad cells back into the network. There is also the requirement for an added layer of wireless infrastructure. The use of micro-cells has therefore been usually restricted to areas where an optical fibre infrastructure is readily accessible, such as in-building.